Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	$ 23,491	$ 13,399
Accumulated other comprehensive income (loss) by component
Other comprehensive loss before reclassifications	(202)	(16)
Amounts reclassified from accumulated other comprehensive loss	143	138
Net current-period other comprehensive income (loss)	(59)	122	$ (183)
Balance at end of period	19,828	23,491	13,399
Defined Benefit Plans
Accumulated other comprehensive income (loss) by component
Amounts reclassified from accumulated other comprehensive loss	143	138
Defined Benefit Plans | Pension Plan
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	(1,731)	(1,895)
Accumulated other comprehensive income (loss) by component
Other comprehensive loss before reclassifications	(195)	39
Amounts reclassified from accumulated other comprehensive loss	128	125
Net current-period other comprehensive income (loss)	(67)	164
Balance at end of period	(2,173)	(1,731)	(1,895)
Defined Benefit Plans | Other Postretirement Benefits Plan
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	(235)	(162)
Accumulated other comprehensive income (loss) by component
Other comprehensive loss before reclassifications	194	(86)
Amounts reclassified from accumulated other comprehensive loss	15	13
Net current-period other comprehensive income (loss)	209	(73)
Balance at end of period	(58)	(235)	(162)
Foreign Currency Translation Adjustment and Other
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	(29)	(60)
Accumulated other comprehensive income (loss) by component
Other comprehensive loss before reclassifications	(201)	31
Amounts reclassified from accumulated other comprehensive loss	0	0
Net current-period other comprehensive income (loss)	(201)	31
Balance at end of period	(230)	(29)	(60)
AOCI Attributable to Parent
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance at beginning of period	(1,995)	(2,117)	(1,934)
Accumulated other comprehensive income (loss) by component
Net current-period other comprehensive income (loss)	(59)	122	(183)
Balance at end of period	(2,461)	$ (1,995)	$ (2,117)
Accounting Standards Update 2018-02
Accumulated other comprehensive income (loss) by component
Amounts reclassified from accumulated other comprehensive loss	(407)
Accounting Standards Update 2018-02 | Defined Benefit Plans | Pension Plan
Accumulated other comprehensive income (loss) by component
Amounts reclassified from accumulated other comprehensive loss	(375)
Accounting Standards Update 2018-02 | Defined Benefit Plans | Other Postretirement Benefits Plan
Accumulated other comprehensive income (loss) by component
Amounts reclassified from accumulated other comprehensive loss	(32)
Accounting Standards Update 2018-02 | Foreign Currency Translation Adjustment and Other
Accumulated other comprehensive income (loss) by component
Amounts reclassified from accumulated other comprehensive loss	$ 0